filesize load error???

So, I'm pretty new to UTAU and sometimes when rendering the song, it renders as "filesize load error" for each note. It doesn't do it for every UTAUloid and every ust. Like, sometimes one UTAUloid can sing the ust and another can't, or one of the usts in a song pack will work and the other won't. I'm not sure if this has already been asked, but it's getting quite annoying.